The present invention relates to a method to assign upstream timeslots to a plurality of network terminals, in a communications network wherein a central station is coupled to the plurality of network terminals via the cascade connection of a common link and respective individual links and wherein the network terminals transmit upstream information to the central station in a time multiplex way over the common link using the upstream timeslots. It is also directed to an arrangement to be used in a network terminal to perform this method, wherein a central station is coupled to the plurality of network terminals via the cascade connection of a common link and respective individual links and wherein the network terminals transmit upstream information to the central station in a time multiplexed way over the common link using the upstream timeslots of a communications network an arrangement to be used in a network terminal.
The invention is further directed to an arrangement to be used in a central station of a communications network to perform this method, wherein a central station is coupled to a plurality of network terminals via the cascade connection of a common link and respective individual links and wherein the network terminals transmit information to the central station in a time multiplexed way over the common link using upstream timeslots assigned by the central station.
Such a method is already known in the art, e.g. from the proposal to the FSAN-OAN (Full Service Access Networkxe2x80x94Optical Access Network) Workgroup entitled xe2x80x98Short Slot Proposalxe2x80x99. This proposal is written by the authors Mark Bridger, Dan Donovan, Paul Welton, and Mike Haynes, and dated Apr. 22-23, 1997. Therein, the central station of a passive optical network (PON) with point-to-multipoint architecture regularly broadcasts so called PLOAM (Physical Layer Operation Administration and Maintenance) cells containing identifiers of the network terminals which are allowed to send a burst in the upstream timeslots of an upstream frame. The central station in the optical network of the mentioned proposal is called the OLT (Optical Line Termination), whereas the network terminals are named ONU""s (Optical Network Units). The contents of such a PLOAM cell as proposed by Mark Bridger et al. is shown in a figure on page 4 of the just cited proposal. The 30 grant messages therein each contain an ONU identifier. Each ONU identified in the PLOAM cell is permitted to occupy one timeslot. By downstream broadcasting this message, all ONU""s in the network become aware of the order wherein they are allowed to occupy upstream timeslots for transmission of data bursts towards the OLT. The upstream timeslots may be standard full size upstream slots with a length of 56 bytes or short slots with a length of 7 bytes. Schemes of the contents of such a short slot and such a standard full size slot are drawn in the figures on pages 2 and 3 of the proposal.
Broadcasting for each upstream timeslot an identifier of the network terminal that is allowed to send an upstream burst in it inevitably involves a high downstream bandwidth occupancy by grant messages. Especially the upstream transmission of short bursts in short timeslots requires a relatively high downstream bandwidth capacity for transfer of overhead information. In networks such as the passive optical network in the cited proposal, the length and transmit rate of PLOAM cells are fixed, and thus the bandwidth available for downstream transmission of grant messages is fixed and limited. Applying the known method for assigning timeslots to network terminals in such networks puts a severe limit on the number of timeslots within an upstream frame, irrespective of the length of these timeslots.
An object of the present invention is to provide a method to assign upstream timeslots to a plurality of network terminals and related arrangements of the above known type, but wherein the downstream bandwidth occupancy to assign a certain amount of upstream timeslots is reduced or, in other words, wherein the amount of timeslots that can be assigned with a certain downstream bandwidth capacity is increased.
According to the invention, this object is achieved by a method to assign upstream timeslots to a plurality of network terminals in a communications network wherein a central station is coupled to the plurality of network terminals via the cascade connection of a common link and respective individual links and wherein the network terminals transmit upstream information to the central station in a time multiplexed way over the common link using the upstream timeslots, wherein the central station downstream broadcasts a group identifier to assign a plurality of the upstream timeslots in a predefined order to network terminals of a group of the plurality of network terminals, the group having a predefined composition and being identified by the group identifier.
The object is also achieved by an arrangement to be used in a network terminal of a communications network wherein a central station is coupled to a plurality of network terminals via the cascade connection of a common link and respective individual links and wherein the network terminals transmit information to the central station in a time multiplexed way over the common link using upstream timeslots assigned by the central station, the plurality of network terminals comprising the network terminal, wherein the arrangement comprises: comparison means, adapted to compare a group identifier, downstream broadcasted by the central station to assign a plurality of the upstream timeslots in a predefined order to network terminals of a group of the plurality of network terminals, the group being identified by the group identifier, with group identifiers of groups of network terminals whereof the network terminal forms part; and control means, coupled between the comparison means and a transmitting part of the network terminal and adapted to control, in case the group identifier downstream broadcasted matches one of the group identifiers, the transmitting part to transmit an upstream burst in a thereby assigned upstream timeslot and to respect the predefined order.
The object is still further achieved by an arrangement to be used in a central station of a communications network wherein the central station is coupled to a plurality of network terminals via the cascade connection of a common link and respective individual links and wherein the network terminals transmit information to the central station in a time multiplexed way over the common link using upstream timeslots assigned by the central station, where the arrangement comprises: selection means adapted to select a precomposed group of network terminals; message generating means, to an input of which the selection means is coupled and which is adapted to generate a broadcast message including a group identifier of the precomposed group of network terminals; and a transmitting part, to an input of which the message generating means is coupled, the transmitting part being adapted to broadcast the broadcast message to the plurality of network terminals to thereby assign a plurality of the upstream timeslots in a predefined order to the network terminals of the precomposed group.
Indeed, no more bandwidth than that used in the known method to assign a timeslot to a single terminal, is used according to the present invention to assign a plurality of timeslots to a plurality of terminals. More precisely, as many timeslots are assigned as there are network terminals in a group. The amount of downstream bandwidth needed thereto is that for transmission of a group identifier and may be considered equal to that needed for transmission of a terminal identifier in the known method. As a result, the downstream bandwidth occupancy to assign an equal amount of timeslots to terminals is reduced with a factor equal to the number of network terminals that composes one group. If, on the other hand, the downstream bandwidth for transmission of grant information is fixed, the same amount of bandwidth can be used according to the present invention to assign an increased number of timeslots. Compared to the known method, the number of timeslots that can be assigned has increased by a factor equal to the number of network terminals composing one group.
It is to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noted that the term xe2x80x98coupledxe2x80x99, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x98a device A coupled to a device Bxe2x80x99 should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
An additional feature of the present invention is the composition is predefined by the central station and communicated from the central station to the plurality of network terminals.
A further feature of the invention is such an arrangement that further comprises grouping means, adapted to precompose the group of network terminals and to assign thereto the group identifier; and communication means, coupled between the grouping means and another input of the transmitting part, the communication means being adapted to generate a message containing information with respect to the composition of the precomposed group.
In this way, when the central station itself composes the groups of terminals, the central station is able to modify the composition of groups whenever this is preferable. When a terminal is de-activated for instance, it is preferred to eliminate this terminal from all groups where it belongs to so that it does not receive any permission anymore to occupy upstream timeslots. It also is preferable to compose a group of most active terminals, so that no timeslot is to be assigned to a little active terminal each time a timeslot is assigned to the most active terminal. Obviously, the composition of this group changes in time.
Another additional feature of the present invention is where the composition is predefined implicitly via a membership-relation between the group identifier and terminal identifiers of the network terminals that form part of the group.
Compared with the above mentioned implementation of the present invention composing the groups implicitly via a membership-relation has the advantage that no additional complexity is required in the central station for composing the groups and communicating the composition of the groups to the network terminals. On the other hand, composing the groups via a membership-relation has the drawback that the groups are not modifiable.
A further feature of the implementation of the present method is wherein the group is composed of a first network terminal whose terminal identifier equals the group identifier, and at least one network terminal with successive terminal identifier.
By composing for example a group with group identifier 1 of the terminals with identifiers 1, and 2, a group with group identifier 2 of terminals with identifiers 2, and 3, a group with group identifier k of the terminals with identifiers k, and k+1, any terminal can determine from its own terminal identifier whether it belongs to a group or not if it receives the identifier of this group. Thus, the terminal does not need to memorize a table of groups where it belongs to. It only has to be aware of the membership-relation between its terminal identifier and the group identifiers of groups where it belongs to.
Also a further feature of the present invention is wherein the arrangement further comprises: memory means with an output coupled to an input of the comparison means, the memory means being adapted to memorize the group identifiers of the groups of network terminals whereof the network terminal forms part.
In this way, a network terminal is capable of memorizing the groups where it forms part of and the table can be adapted whenever the composition of groups has changed. The use of such a table, as already indicated in the previous paragraph, is avoided in implementations of the present invention wherein each terminal can determine the groups where it forms part of from a simple membership-relation.
Yet a further feature of the present invention is where the memory means further is adapted to memorize for each group of the groups of network terminals whereof the network terminal forms part, an index of the network terminal in the group, i.e. an integer number defining the predefined order for the group.
In this way, a network terminal is aware of the predefined order to be respected by the terminals composing a single group in case this order is not derivable from a membership-relation or any other rule known by the terminal. From the index, the network terminal can determine in which timeslot it is allowed to transmit a burst and from which starting time it is allowed to transmit this burst if the timeslots have fixed lengths. The terminal moreover can keep track of any changes in order between terminals in one group if, for some unspecified reason, this would be preferable.
Still a further feature of the present invention is where the memory means further is adapted to memorize for each group of the groups of network terminals where the network terminal forms part of, time reference information defining the starting time of an upstream timeslot wherein the network terminal is allowed to send information and where the message generating means further is adapted to embed in the broadcast message time reference information defining the starting time of the upstream timeslots assigned to the network terminals of the precomposed group.
In this way, a network terminal is also capable of determining the starting time from which it is allowed to transmit a burst in case the timeslots have variable lengths. This is for instance so in the co-pending European Patent Application entitled xe2x80x98Time Slot Management Method and Main Station and Substation Realizing such a Method and Time Division Multiple Access Network Including such a Main Station and such a Substationxe2x80x99, filed at even date by the same applicant. The time reference information for instance can be the offset from the start time of an upstream timeslot to the start time of an upstream frame, or the offset of the start time of a short timeslot to the start time of a standard timeslot whereof a short timeslot forms part if only the short timeslots are made modifiable in length
Still a further feature of the present invention is where the memory means further is adapted to memorize for each group of the groups of network terminals where the network terminal forms part of, a timeslot length defining the length of an upstream timeslot wherein the network terminal is allowed to send information and where the message generating means further is adapted to embed in the broadcast message timeslot lengths for the plurality of upstream timeslots assigned to the network terminals of the precomposed group.
In this way, the central station can adapt the length of a timeslot assigned to a specific terminal whenever this is desired, and the network terminal is capable of determining the length of the timeslot in which it is allowed to transmit an upstream burst.